. Introduction

Plastic was created to enhance human living conditions, but it now poses a serious threat to the planet's safety and environment (1,2). Artificial materials made of high molecular weight molecules, known as polymers, either naturally occurring or synthesized, are referred to as "plastics". Plastics have been widely employed in a variety of industries because of their affordability, adaptability, strength, durability, and lightweight(3,4). Additives are added to plastic during production to give it particular qualities (5). Polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS), polyvinyl chloride (PVC) and others are major commercial plastics on the market (2,6,7).Numerous sources contribute to plastic trash's direct and indirect accumulation in aquatic ecosystems (8). According to (7), a high yield and low recovery indicate that a significant amount of plastics end up in the environment. Plastic remains in the aquatic environment because it is designed to be long-lasting. Plastic polymers can, however, be broken down gradually by heat, oxidation, light, hydrolysis or microbes (such as Bacillus cereus, Micrococcus sp., or Corynebacterium) (9). Plastic splits and fragments due to thermal stress caused by temperature changes. Examples of plastic degradation and MP generation include the disintegration of plastic bags and packaging containers, the dispersal of synthetic fibers from garments and the breaking up of plastic bottles and containers (10).Synthetic substances with a high molecular weight that have been micronized into plastic particles smaller than 5 mm are known as microplastics (11). Primary and secondary microplastics are distinguished by whether the particles were created with that size in mind (primary) or are the consequence of larger objects breaking down (secondary). Plastic nanoparticles utilized in various industrial processes, plastic powders used in molding, micro-beads used in cosmetic formulation and industrial "scrubbers" used to blast clean surfaces are examples of primary microplastics. Secondary microplastics are formed when bigger plastic products are fragmented and weathered. This can occur while materials like paint, tires and textiles are being used or after they have been released into the environment (12,13).

Figure 1: Sources, fate and effects of microplastic in aquatic environment.

Organisms interact with microplastics at different trophic levels and interaction can happen through a variety of pathways. While microplastics present in the environment (water or sediment) directly expose organisms, those ingested by prey may indirectly contaminate predators through trophic transfer (14,15). Fish is considered as a good source of protein all over the world. A recent WorldFish and Indian Council of Agricultural Research study on fish consumption patterns shows that from 2005 to 2021 there was an 81.43% increase in per capita fish consumption in India, along with a 32% growth in the fish-consuming population (16). Water bodies are being polluted by microplastics which ultimately affects the life of aquatic organisms including fishes. Microplastics have a variety of effects on fish health, either by themselves or when combined with other environmental pollutants. Fish exposed to these pollutants experience reductions in growth, oxidative stress, immunotoxicity, neurotoxicity, inflammation, organ damage, physical harm, and behavioral changes (17). According to (13), the discovery of microplastics in every aquatic compartment has led to their recognition as a ubiquitous contaminant, posing a threat to both aquatic life that consumes them and humans who consume fisheries products. This review aims to examine the effects of microplastic exposure on fish physiology, focusing on accumulation, organ function, and potential implications for reproductive health.

2. Materials and Methods

We conducted a systematic literature review. Search engines such as Google Scholar, NCBI PubMed, ResearchGate, ScienceDirect, and SpringerLink have been used to search literature relevant to the topic. The keywords, such as “microplastic effects on fish," "fishes," "physiology," "reproduction," and “cytotoxicity in fish,” were used in various combinations during the literature search. Most of the publications searched were in environmental science, agriculture and biological sciences, and earth and planetary sciences. After screening 200 papers, 74 were chosen for reference by eliminating those that were unrelated to our study.

2.1 PHYSIOLOGICAL EFFECTS OF MICROPLASTIC ON FISH

With the prevalence of small-sized particles, such as microplastics in everyday products (ingredients in many daily use items, cosmetics, packaging, and so on) and nanoparticles in drugs, the biomedical field, and various formulations, there are growing concerns about their negative impact on aquatic life. Their small size makes them easily consumed by a wide range of species, from zooplankton to larger marine mammals. Because these pollutants have the potential to directly or indirectly penetrate the environment and contaminate soil, water, and air, it is now important to comprehend how they interact and impact the aquatic biota in their natural habitat (18). Fish are the primary vertebrate organisms utilized for monitoring microplastics (MPs), likely due to their higher exposure levels to these particles. MPs can enter fish through various routes, including their gills and the consumption of food and water (19, 20).

Figure 2: Physiological alterations in fish after microplastic exposure.

Toxicity and cellular damage:

MPs are tiny pieces of plastic that are commonly found in aquatic ecosystems all around the world. There have been reports of physiological problems in fish exposed to MPs, including oxidative stress, neurotoxicity, and immunotoxicity (21). Particle toxicity, inflammation, and oxidative stress all contribute to cytotoxicity (13,22). Severe organ malfunction and disease may arise from cell toxicity brought on by environmental pollutants and medications. According to recent research, the primary mechanisms of cell toxicity include DNA damage, mitochondrial malfunction, oxidative stress brought on by ROS, and excessive NO generation (23).MPs were found to be cytotoxic due to oxidative damage and inflammation (24). Microplastics can induce oxidative stress by releasing reactive oxygen species (ROS) produced during the inflammatory response and oxidizing substances (like metals) that were previously adsorbed on their surfaces (13,25).The interaction between MPs and cellular components can affect cell signaling, which in turn can activate the processes of autophagy, apoptosis, and proteolysis. Oxidative stress causes metabolic changes in fish tissues. MPs' distribution and accumulation in fish liver and gills have a significant impact on tissue toxicity (26). "Aged" microplastics can activate the antioxidant defense mechanisms and have an impact on the cellular health of freshwater fish species Percafluviatilis(27). Long-term exposure to microplastics can cause immune cell dysfunction, necrosis, chronic flogosis, and cell proliferation in organisms (9,13).In zebrafish, PE-MPs can significantly alter the activity of Na+/K+-ATPase and antioxidant enzymes (28). Both microplastics and mercury independently induced neurotoxicity via acetylcholinesterase (AChE) inhibition, lipid peroxidation in neural and muscular tissues, and alterations in the activity of energy-related enzymes lactate dehydrogenase (LDH) and isocitrate dehydrogenase (IDH) (29,30). The association between zinc oxide nanoparticles (ZnO-NPs)andpolyethylene microplastic (PE-MPs) exacerbates and raises serious health difficulties in aquatic forms. Therefore, for the sake of environmental safety, the release of these particles must be controlled. More research is needed to better understand the toxicity mechanism of the interaction in fish (18).

Effects on the immune system:

Microplastics or nanoplastics may change organismal defense mechanisms, which could disrupt innate immune responses in fish populations. Fish larvae may experience harm to their sense of smell as a result of an immunological reaction brought on by microplastic pollution. Furthermore, it has been demonstrated that POPs are absorbed by plastic fragments present in marine environments. Therefore, absorbed ambient chemicals, plastic-associated compounds, and particle toxicity can impact the immune system (31). The immunological, digestive, and reproductive systems are all affected by virgin MPs and nanoparticles (NPs), which also cause intestinal dysbiosis and may have generational implications (32). Fishes may experience stress from microplastics, which could interfere with their natural defenses. They can also cause immune cells to have less phagocytic activity, less cell viability, and lysosomal membrane degradation (33,34).Exposure to microplastics can also influence the activation of pro-inflammatory cytokines and induce inflammation (34,35). MP accumulation and goblet cell growth was discovered in the stomach of a young guppy following a 28-day exposure to MPs(36). MPs lowered the function of digestive enzymes and raised immune cytokine levels in the gut.High quantities of MPs cause cellular immunological stress in yellow catfish, and hypoxia intensifies these effects on immune parameters (37). According to(38), microplastics can trigger an inflammatory response by upregulating pro-inflammatory cytokines. They demonstrate that MPs may affect the immunological response and antioxidant system of GIFT (Genetically Improved Farmed Tilapia), a strain of Nile tilapia (Oreochromisniloticus). Fish exposed to microplastics showed elevated brain AChE activity and lipid oxidative damage to their brain, muscles and gills (39).

Effects on the respiratory and circulatory system:

The degree of direct ionoregulatory disruption caused by microplastics varies according to the type, size, concentration, and exposure schedule of the microplastics (40).Compared to the specimens that tested negative for MP ingestion, the gills of the MP-ingested animals displayed greater LPO levels and a higher index of lipid peroxidation damage (39,41).Damage caused by gill lipid peroxidation might have negative consequences, such as impaired respiration and xenobiotic biotransformation (41,42).Although microplastics have no direct effect on C. gariepinus survival, their buildup in fish tissues increases the fish's opercular respiratory rate (ORR) and decreases its swimming speed, which may have an impact on the fish's capacity for foraging and make them more vulnerable to predators(43). In (44), histological analyses were conducted after exposing early juvenile tilapia (Oreochromis niloticus) to MPs. Complete lamellar fusions, lifting of the epithelium, shortening, and degeneration of secondary lamellae, hyperplasia, blood vessel dilatation, and congestion, and MP deposition between primary lamellae were all observed in the gill tissue.

Polyethylene (PE) microplastics accumulate in certain organs, affecting the hematological parameters, plasma components, and antioxidant response of juvenile P. fulvidraco. Every physiological change brought on by acute exposure to PE-MPs was concentration-dependent (45). After zebrafish were exposed to sublethal quantities of Polystyrene microplastics (PS-MPs), there was a decrease in heart rate, an increase in oxidative stress through the stimulation of associated parameters that led to autophagy and apoptosis, metabolic changes in the heart, and a decrease in fish activity. Cardiotoxicity has been noted in fish, and several forms of plastic have been shown to accumulate in the heart, indicating a trophic transfer through the bloodstream. The circulatory system of a fish may be impacted by plastic pieces at various stages of its life cycle. Indeed, morphological modifications and variations in heart rate were noted in embryos and larvae following maternal transfer (46).

Effects of Microplastic on various fish species:

 Effects on the digestive system:

Satiety is one of the initial impacts of MP consumption, which alters the body's eating habits. As a result, a decrease in food intake is typically seen (2,70–73).

Exposure to microplastic produced varying degrees of detrimental effects on fish liver function, including oxidative stress from free radicals, liver lipid metabolism problems, immune system impairment, and liver tissue damage and inflammation (74).According to (57) microplastic particles of PA, PE, PP, PS, and PVC damaged D. rerio's digestive tract by causing enterocyte splitting and villi cracking.MPs can harm fish physically by causing internal abrasions and digestive tract blockages. Poor health, poorer digestion, and decreased vitamin absorption can result from this. It was discovered that MPs could interfere with fish metabolism and hormonal balance (18).MPs can accumulate in fish's digestive tracts as a result of absorption, leading to gastrointestinal tract perforation or a false sense of satiety that makes them hungry. These MPs have detrimental effects on fish's bodies and physiologies (75).Although microplastic exposure may partially impact intestinal shape and the intestinal microbial community's recombination process, it had no discernible effect on the diversity or composition of the intestinal microbial community(68).However, microplastics can upset intestinal flora, changing the ratio of beneficial bacteria to dangerous ones (34,76).

Different MP concentrations caused metabolic problems and hepatocellular vacuolization in grass carp (58). In the digestive tract of zebrafish, microplastics cause fat accumulation and liver inflammation (59). MPs usually interfere with the liver's ability to metabolize energy, lipids, and glucose (69). Physiological issues of goldfish (Carassiusauratus) are influenced by the size of microplastics, particularly polystyrene(47). Histological damage was found in the fish's liver, intestine, and gills after exposure to the microplastic. Both the dosage and the size of the microplastic were found to have an impact on the severity of these lesions. Omnivorous and carnivorous fish are more likely to ingest microplastics in freshwater and marine environments, according to contamination based on eating practices(77). In freshwater fish, polyethylene terephthalate was the most common plastic polymer, whereas in marine fish, Polyethylene was the most common.While MP particles with a small size and high concentration caused more severe oxidative stress and hepatic congestion, MP particles with a big size and high concentration caused more severe intestinal damage and less weight gain (53).

Effects on reproductive health:

Fish are severely harmed by MPs, which affect their development, survival, and ability to reproduce. Compared to freshwater fish, marine fish exhibit a higher tolerance to MPs in terms of growth and reproduction(78). In (48), the effects of polystyrene microplastics (PS-MPs) on survival, reproduction, body weight, and gene expression in Japanese medaka was investigated.During the first few days of the reproductive season, medaka fish (Oryziaslatipes) showed MP-related differences in fecundity, fertilization rates, and the number of gravid females. Over the next few days, these numbers returned to their typical rates. There were no discernible impacts of polystyrene MP exposure on the success of the progeny (51). Microplastics result in delayed gonadal maturation, diminished body mass, and hastened female sexual maturity in marine medakas. The histological study indicated modifications in the structure of gill lamellae and abnormalities in the gonads (51,64,65). Exposure to PS-MPs negatively impacted fertilization and significantly changed the gonadosomatic index (GSI) in zebrafish(52). The female hypothalamus-pituitary-gonadal (HPG) axis was negatively regulated by microplastics. Incubation was delayed, and the offspring's body length, heart rate, and hatching rate all decreased when their parents were exposed to 20 μg/L of microplastics(64).

The harmful effects of microplastic on Nile tilapia (Oreochromesniloticus) was investigated and it was found that the fish subjected to MP developed testicular, histological, degenerative, and testis-ova alterations(54). In adult zebrafish, acute exposure to microplastics such as PE can disrupt oogenesis, change the aryl hydrocarbon receptor (AHR) pathway, and result in neurological or behavioral problems(60).Males treated with microplastic had significantly higher mortality, shorter standard length, smaller body area, fewer sperm bundles and sigmoid displays, and lower levels of sexual interest(79). 1% of MPs recovered from the sea after natural weathering affected gonadal development, digestion, and reproductive endocrinology (i.e., successful fertilization and egg production) in Atlantic cod from early maturation to spawning(50).Microplastics made of polythene significantly harm population sustainability and affect reproduction, claim (66). The effects of parental exposure to polystyrene microplastic on zebrafish spawning, fertilization, survival, and offspring growth were investigated by(61). Polystyrene microplastics (PS-MPs) damage the female reproductive system and the processes that cause reproductive toxicity(80).
According to (63), fish exposed to pristine or contaminated polyethylene microplastics in their diet for an extended period may experience problems with growth and reproduction and affect future generations.MP contamination in the aquatic environment can have a direct effect on the reproductive function of freshwater fish, including the African catfish. MP consumption may contribute to reproductive stress, and reproductive toxicity in male catfish. For example, testicular injury, reduced sperm quality and viability and suppressed hormonal profiles were observed (49). Species-specific reproductive responses to microplastic exposure varied, but all showed a significant decrease in fertility, sperm swimming speed, gamete and oocyte quality and offspring quality (33).

3. CONCLUSION

In conclusion, we pinpoint areas of unmet knowledge that require more study, such as examining the potential consequences of microplastic exposure on future generations and measuring the positive and negative interactions between MPs and other ecological contaminants. This increased understanding of the possible consequences of MPs on aquatic wildlife is expected to help policymakers design mitigation policies to preserve aquatic species. More long-term studies are needed to assess chronic effects on fish populations and ecosystems.